Popularization of the Internet has made it possible for any one to easily obtain necessary information via a computer. Especially, in these days megabit-class high-speed lines such as xDSL (Digital Subscriber Line) and FTTH (Fiber To The Home) are becoming popular in general households, and contents which used to be distributed through a physical medium such as a CD-ROM tends to use a digital medium for on-line distribution via the high-speed line.
One typical example of the service taking advantage of the feature of the high-speed Internet may be distribution of a moving image. In general, there are two distribution methods for the moving image; a downloading type for replaying the moving image after downloading all the moving image data to a personal computer, and a streaming type for replaying the moving image in real time while downloading the moving image.
In the case of the former, namely the downloading type, on one hand the moving image can be advantageously replayed without a pause because the moving image data is once stored in the personal computer, and on the other hand, since the management of the distributed moving image content is transferred to the user side, the moving image content may be secondarily used by unauthorized copying and the like to violate the copyright of the moving image content.
In the case of the latter, namely the streaming type, while the secondary use described above can be prevented since only a key file including information on the storage location of the moving image content remains in the user terminal but not the actual moving image content remains therein, it is necessary to make the speed of the moving image data transmission faster than the speed of replaying the motion. Namely, when the transmission speed is slower than the replaying speed, the moving image may be replayed intermittently due to the data buffer to make the user uncomfortable.
As described above, either of the downloading type and the streaming type has good and bad points in distribution of the moving images. An example of a moving image distribution method making good use of the advantages of the downloading type and the streaming type is described in patent document 1. Patent document 1 describes a method of dividing a moving image content into two not-producible moving image files and decrypting the same with a decryption key.
This method enables the user to view the moving image smoothly like in the downloading type by taking the divided moving image data into a personal computer, receiving the decryption key from a service provider server upon authorization, and decrypting the moving image data.
However, while the moving image distribution method described above is suitable for the downloading type distribution, the following problem remains when the method is applied to the streaming type distribution. Namely, since the moving image file is divided simply along the time axis, the method described above can distribute the moving image file in an not-producible state, but the size of the moving image file is not in consideration.
As described above, since the streaming replay distributes a lighter moving image file to the user terminal first and distributes a heavier moving image file on-line, the server has a heavy load during the distribution, and the load increases as the number of access to the server increases. Therefore, since both files have large capacities in the method described above, it is not possible to reduce the load applied to the server during the distribution.
Primarily, since the downloading type distribution distributes all the moving image files to the user terminal, the number of the files stored in the hard disc (recording device) in the user terminal is limited. Also from such a view point, the streaming type of the moving image distribution method recently tends to be preferred to the download type.
Also, the decryption key is distributed to the user terminal for replaying the moving image content, but a copy data of the decryption key (namely, a concave data assuming the decryption key data as a convexity) may remain in the recording device (such as a hard disc) in the user terminal, and the decryption key may be copied based on the copy data,
Patent document 1: Japanese patent publication No. 2002-353957